1. Field of the Invention
The present invention relates to an ink set having at least three kinds of black inks different from one another in carbon black content, an ink cartridge, a recording unit, and an ink jet recording apparatus each using multiple inks constituting the ink set, and an ink jet recording method and an image forming method each using the ink set.
2. Related Background Art
An ink jet recording method involves: ejecting small ink droplets; and applying ink to a recording medium such as paper to perform recording. In particular, an ink jet recording method of a mode in which an electrothermal transducer is used as ejection energy supplying means and thermal energy is applied to ink to generate air bubbles for the generation of droplets (a thermal ink jet recording method), a recording head can be easily provided with multiple openings at a high density, and a high-resolution, high-quality image can be recorded at a high speed (see, for example, Japanese Patent Publication Nos. S61-59911, S61-59912 and S61-59914).
Such ink jet recording techniques have recently found use in an expanded variety of applications. Accordingly, ink that can find use in a variety of applications has been desired. In particular, pigment ink using a pigment such as carbon black as a coloring material has been used for applications where emphasis is placed on fastness property. Even such applications of the pigment ink require not only the formation of a beautiful, colored image but also the formation of a monochrome image with beautiful gradation or a beautiful halftone.
The use of multiple black inks whose carbon black contents differ in a stepwise manner has been proposed for meeting such requirements as described above (see, for example, Japanese Patent Application Laid-Open Nos. H11-48502 and 2003-238857). The proposition describes that the use of such black inks can improve color reproducibility for a dark color of a shadow portion or the like or gray gradation, can stabilize a gray balance, and can dissolve metamerism (a phenomenon in which two colors different from each other in spectral reflectance appear to be the same color under a specific light source).
In the present invention, ink having an ordinary carbon black content is referred to as black ink, and ink having a small carbon black content is referred to as light black ink.
In addition, there has been proposed the use of at least three kinds of inks different from each other in pigment content as a set for the purposes of: reducing the reddish coloring property and yellowish coloring property of an image; reducing a golden gloss phenomenon; reducing a phase shift; improving the stability of a gray balance; and alleviating metamerism (see, for example, Japanese Patent Application Laid-Open Nos. 2004-027127, 2004-099657, 2004-225036 and 2004-250519). Furthermore, a proposal in which two or more kinds of light black inks are added to an ordinary ink set describes that an alleviating effect on the above-mentioned problem in terms of a tint or a hue is obtained by: causing each of these black inks to contain a coloring material except carbon black; providing an ink set having a combination including light cyan or light magenta; or providing an ink set having a combination including ink of each color (red, green, violet, or orange) or an ink set including a resin emulsion.
The reason why an ink set is obtained by adding two or more kinds of light black inks to an ordinary ink set in the above-described prior art is based on the following idea. That is, as compared to the case where only one kind of light black ink with regard to a carbon black content is added to an ink set, the case where another kind is further added so that inks can be classified into three kinds of inks having the same color tone and different from one another in pigment content is more advantageous for an improvement in gradation of an image to be obtained by means of such inks and a reduction in granularity of the image. In addition, in the above-described prior art, a specific range of a carbon black content in each of the inks having three kinds of pigment contents is specified as described below. A carbon black content in black ink having the smallest carbon black content is preferably 0.01 to 0.4 mass % with respect to the total mass of the ink. A carbon black content in black ink for an intermediate gray level having an intermediate carbon black content is preferably 0.4 to 1.5 mass % with respect to the total mass of the ink. A carbon black content in dense black ink having the largest carbon black content is preferably 1.5 to 10 mass % with respect to the total mass of the ink.